Iron chlorosis is a nutritional plant disorder which results in a decrease in the amount of chlorophyll and in the yellowing or whitening of normally green plant tissue. It is a widespread problem in agriculture that affects the development of many crops provoking a decrease yield.
Attempts have long been made to correct this iron deficiency by feeding the plants with iron in assimilable form, originally in the form of ferric salts. Ferric salts act to a certain extent by foliar penetration, but are substantially ineffectual when used for soil treatment. It is through the roots that the penetration of iron is at its most effective in controlling chlorosis. In this connection, simple iron salts have been replaced by iron chelates which are hydrosoluble complexes consisting of an iron atom attached to several points of a single organic molecule known as chelates or co-ordinates. Some of these products known as hexadentates, such as complexes of iron with ethylene diamine tetracetic acid (EDTA) or with diethylene triamine pentacetic acid (DTPA) or with N,N′-ethylene-bis-(hydroxyphenyl)glycines (EDDHA), have been successfully used against chlorosis.
N,N′-ethylene-bis(2-hydroxyphenyl)glycine (o,o-EDDHA) is often described as one of the most favorable compounds for complexing metals, and being most suitable for the use as a plant nutrient or fertilizer because the complexation affinity to iron is particularly strong. The ferric chelate of o,o-EDDHA shows a coordination number of 6 and is widely used for agrochemical purposes as a plant nutrient or plant fertilizer, and for the treatment of chlorosis in crop plants.
The N,N′-ethylene-(2-hydroxyphenyl)-glycinyl-(4-hydroxyphenyl)glycine (o,p-EDDHA) is also known in the art. Contrary to o,o-EDDHA, in the o,p-EDDHA isomer one of the hydroxy groups is in the para-position of the benzene ring. For this reason, the ferric chelate of o,p-EDDHA shows a coordination number of 5 and its complexation affinity to iron is weaker than that of o,o-EDDHA. This compound has often being regarded as an undesired by-product being incurred by the manufacturing process of o,o-EDDHA.
Gómez-Gallego et al. (c.f. Chem. Eur. J. 2005, 11, 5997-6005) propose a model for explaining the reduction of the Fe(III)-o,o-EDDHA chelate by a ferric chelate reductase. According to it, the reduction of said chelate does not take place on the complex in the octahedral closed form, wherein the 6 chelating points of o,o-EDDHA are coordinated to the Fe(III), but on a hexacoordinate open species formed at the acid pH of the rhizosphere. As showed in Scheme 5 of said report, one of the hydroxy groups in the ortho-position would not be coordinated with the Fe(III), thus generating a vacant coordination site that would be filed with a water molecule. The complex Fe(III)-o,p-EDDHA, wherein one of the hydroxy groups is in the para-position, would already be in the required open form and could be reduced directly by the enzyme.
WO2005/095305 discloses isomeric mixtures comprising both (o,o-EDDHA) and (o,p-EDDHA), wherein the molar ratio of (o,p-EDDHA) to (o,o-EDDHA) is higher than 0.8:1, as nutrients for agricultural crop plants and agents for treatment of chlorosis in such plants.